PROTEINS, a large group of organic com pounds, of which the white of egg or oval bumin, may be taken as a typical example. They make up the principal bulk of the solid matter of all animal cells and tissues, and are found in various parts of the structure of plants, especially in seeds. The proteins found in nature are 40 or 50 in number, mostly amor phous and indiffusible, and possess a complex chemical structure, whose nature is not clearly understood. They appear as non-crystallizable compounds of carbon, hydrogen, nitrogen, oxygen, generally, but not always, sulphur, and sometimes phosphorus i and occurring in a solid viscous condition, or in solution. Iodine is a usual constituent of the proteins found in marine animals. The different members of the group present differences in physical, and to a certain extent in chemical, properties; they all possess, however, certain common chemical re actions, and are united by a close genetic rela tionship. Thus, carbon is present in amount varying only from 50 to 55 per cent; hydrogen, from 6.9 to 72 per cent; oxygen, from 19 to 24 per cent; nitrogen, from 18 to 19 per cent ; and sulphur, from 0.3 to 2.4 per cent—keratin of human hair, being a notable exception, having a sulphur content of 5 per cent. Proteins occur mg in the vegetable kingdom are built up from simpler compounds; but the animal system is believed to be incapable of performing a syn thesis of this kind, and the proteins that occur in the animal body are believed to be derived wholly by a modification of those that are taken into the system in the form of food. The pro teins in the food are largely converted, by the action of such digestive ferments as pepsin and pancreatin, into soluble substances known as pept o nes,'" and these, after absorption through the intestinal walls, are (in large measure) transformed into the proteins that normally occur in the muscles and other tissues of the animal. Some of the protein portion of the food is, however, apparently in the course of digestion and assimilation, into glycogen and fat. In the changes which are inseparably associated with the life of the animal, the proteins that are stored in the body (and especially those in the muscular and nervous system) undergo a process of oxida tion, or slow combustion, by which they are transformed into other compounds that are much less highly organized; the ultimate prod ucts of this oxidation being mainly water, car bon dioxide and urea.
The proteins, as a class, are so similar to one another in their ultimate composition, that Gerhardt believed them to consist mainly of one fundamental albumen-like substance, min gled, or perhaps chemically combined, with varying amounts of inorganic matter. This
view is not in favor among the chemists of to day.
Efforts to determine the molecular weights of the proteins have been many, and not at all satisfying. The results obtained, while quite indefinite, seem to prove that the molecular weights of the proteins are very large as com pared with those which have been determined for other substances. For example, the molec ular weight of egir-albumen, as obtained by the freezing-point method, is given as 14,270; that of hemoglobin, as determined by the iron con tent method, as 14,000.
Chemically, proteins exhibit a wide variance, some being distinctly acid and others decided bases which form strong salts with strong acids. The most of them, however, are neither markedly acid nor basic, but react toward acids as bases and as acids toward bases. Re cent investigation of the proteins has been chiefly along the line of hydrolysis, accom plished by heating them with concentrated hydrochloric acid or strong sulphuric acid. A large number of amino-acids have thus been obtained from the simple proteins, but with the very unsatisfactory disappearance of about 30 per cent of the protein substance treated.
Though the proteins are mostly amorphous, a few are known that are capable of crystalli zation, notably some which have been obtained from the seeds of hemp and the pumpkin, from Brazil nuts and from castor-oil beans. Some other proteins which are not found in nature in a crystallized form can be obtained in that phase by special treatment. Thus, if egg-albu men is precipitated from its solution by ammo nium sulphate and acetic acid, and allowed to stand, it gradually crystallizes. All the pro teins are insoluble in absolute alcohol, but many of them are soluble in water, or in dilute saline solutions. Dissolved proteins all rotate the plane of polarization of polarized light to the left, the extent of the rotation varying with the nature of the dissolved protein. Dissolved pro teins are not diffusible, as a rule, and mineral matters with which they may be mixed can, therefore, be removed by placing the solution in a vessel which is separated into two parts by a vertical porous membrane, pure water being placed in one compartment, while the solution to be dialyzed is placed in the other. The mineral salts will diffuse through the por ous membrane, while the protein remains in the compartment in which it was originally placed. By renewing the water in the second compart ment from time to time, the dialysis may be caused to proceed until practically the last trace of mineral matter has been removed from the protein. The peptones, however, are them selves diffusible, and the proteoses (or "albu minosesp) are so to a lesser extent.